The present invention relates to a solar heat collector.
There are generally two kinds of solar heat collectors in the art: one wherein a proper vehicle (for example, water) is supplied and circulated within a rectangular box and the other wherein a narrow copper tube is inserted into a cylindrical vacuum glass tube for passage of a proper vehicle. When it is desired to achieve air-conditioning through the utilization of solar heat energy, the latter vacuum tube type is the only effective way in elevating the temperature of the vehicle to a desired value. Accordingly, the vacuum tube type of solar heat collectors seems to be more promising but has still a variety of problems especially in conjunction with manufacturing techniques and operating life.
More particularly, reference is now made to FIGS. 1 and 2 to give a better understanding of these aspects. In FIGS. 1 and 2, there are illustrated an outer glass tube 1 which serves as a transparent cover and a casing and a cylindrical fin structure 2 which assumes a selective absorbent overlaying at least on its outer surface and is made of typically aluminum by a wellknown extrusion molding. A heat-collecting pipe 3 made typically of copper and in heat conducting contact with the cylindrical fin structure 2 provides a passage for the vehicle which is to be heated by removing from the cylindrical fin structure 2, the heat absorbed therein. A sealant 4 is provided at one end of the outer glass tube 1. In order to tightly secure the sealant 4 to the outer glass tube 1, it is desirable that the sealant 4 be made of a material which has substantially the same coefficient of thermal expansion as that of the glass tube 1, for example, iron-nickel-chromium alloy and be bonded to the outer glass tube 1 by the use of a proper adhesive such as low melting point glass frit. Furthermore, the sealant 4 is attached to the collecting pipe 3 through wax. The outer glass tube 1, combined with the sealant 4, forms a vacuum chamber. A vacuum cavity 5 is defined by the outer glass tube 1 and the sealant 4 and brought into a vacuum state with the aid of a vacuum pump connected to a chip tube (not shown). A spacer 6 made of a ceramic material, for example, is provided to secure the cylindrical fin structure 2 in place in the interior of the outer glass tube 1. Since the heat collecting pipe 3 is off center of with respect to the outer glass tube 1, the force resulting from thermal expansion does not act evenly on the sealant 4 and eventually destroys part of the bonding section even though measures are taken to absorb such an uneven force. It is therefore necessary that the glass 1, the sealant 4 and the adhesive 7 have substantially the same coefficient of thermal expansion to maintain a tight and lasting adhesion between elements. A proper sealant material may be an Fe-Ni alloy and a 426 alloy in conjunction with soda glass and Kover in conjunction with class II hard glass. A desirable adhesive may be a low melting point glass having a coefficient of thermal expansion equal to that of the glass tube and the sealant. However, in the event that a metallic material such as a 426 alloy and Kover is exposed to the atmosphere, it will become rusted and corroded. For these reasons such material is disadvantageous for use in solar heat collector assemblies. Anti-rust treatment seems necessary for surfaces of these metallic materials and is actually achieved by plating or depositing a rust proofing coating thereon. Such treatment is however less reliable for a prolonged period of time.
While the outer glass tube 1 is sealed with the metallic sealant 4, a considerable amount of heat is liberated from the sealant 4 and the heat-collecting pipe 3.